1. Field of the invention
The present invention relates to a liquid crystal display apparatus in which a liquid crystal layer is driven by a large number of picture element electrodes disposed into a matrix.
2. Description of the prior art:
In an active matrix type liquid crystal display apparatus, a liquid crystal layer is put between a picture element electrode substrate and an opposite electrode substrate so as to form a liquid crystal display cell, the picture element electrode substrate having a large number of picture element electrodes disposed into a matrix on an insulating transparent substrate. Each picture element electrode connects with a function element for applying predetermined voltage. A thin film transistor (TFT), a metal-insulator-metal (MIM) element, a transistor, a diode, a varistor, or the like, is used as the function element connected to the picture element electrode.
An insulating protective film is formed on the entire surface of the picture element electrode substrate except for areas to be connected to external terminals or the like, in order to prevent each picture element electrode and each function element from being mechanically and/or chemically damaged and a leakage current from flowing between the respective picture element electrodes. On the insulating protective film an orientation film is laminated for orienting liquid crystal molecules at the liquid crystal layer, the orientation film being formed by rubbing-processing, for example, a film of polyimide resin.
On the opposite electrode substrate opposite to the picture element electrode substrate across the liquid crystal layer is disposed an opposite electrode. On the entire surface of the opposite electrode is formed an orientation film, which is formed in the same way as on the picture element electrode substrate. The orientation films on the picture element electrode substrate and the opposite electrode substrate enable twist orientation of liquid crystal molecules.
The Japanese Laid-Open Patent Publication No. 62-296123 discloses a display apparatus using a polyimide resin as an insulating protective film on a picture element electrode substrate. In the display apparatus, the insulating protective film of a polyimide resin is laminated not on the entire surface of each picture element electrode, but on part only thereof. The insulating protective film and picture element electrodes are covered with an orientation film of a polyimide resin. The polyimide resin film used for the insulating protective film is defective in that it is rough membraneous that tends to create pinholes therein and high in moisture permeability which tends to leak an electric charge. Therefore, such a polyimide resin film, when used as the insulating protective film for protecting the function element, such as TFT, must be 2 .mu.m or more in thickness. In the above-mentioned Publication, the polyimide insulating protective film is about 2.5 .mu.m in thickness. Since the liquid crystal layer at the liquid crystal display apparatus is usually about 10 .mu.m in thickness, when the polyimide insulating protective film covers the entire surface of picture element electrode, the impedance of the film corresponds to 1/2 to 1/3 of the impedance of the liquid crystal layer. In the liquid crystal display apparatus having the insulating protective film of such impedance, voltage loss at the insulating protective film is remarkable, therefore a high voltage must be applied between the picture element electrode and the opposite electrode in order to apply the predetermined voltage to the liquid crystal layer. Such a high voltage will increase the load on the function element, such as TFT, and on wirings, and so remarkably reduce a life span of the liquid crystal apparatus. With the liquid crystal apparatus in the aforesaid publication, if the polyimide insulating protective film is removed from the picture element electrode so as to enable the liquid crystal layer to be driven by a relatively low voltage, the above problem, will be solved.
In the liquid crystal apparatus disclosed in the aforesaid Publication, the polyimide insulating protective film laminated on part of the picture element electrode must be made large in thickness. Therefore, the impedance of the insulating protective film becomes large, so that, when voltage is applied to the picture element electrode in order to drive the liquid crystal layer, a voltage to be applied to the liquid crystal layer in the area of the picture element electrode on which the polyimide insulating protective film is laminated becomes lower than that of the area of the picture element electrode on which the polyimide insulating protective film is not laminated. Thus, contrast unevenness is caused in the picture element driven by one picture element electrode, thereby creating the problem in that the display quality remarkably lowers.
When a polyimide resin is used for the orientation film for the liquid crystal layer, internal polarization is apt to occur between the orientation film and the picture element electrode. The mechanism of the generation of internal polarization is not yet known in detail, but it is assumed to be as follows: The polyimide resin has the following structure with a large number of polar groups. ##STR1##
Accordingly, polar molecules or ions are specifically absorbed into the polyimide resin orientation film or onto the surface thereof. It is considered that, since an electric double layer is formed between the absorbed polar molecules or ions and the picture element electrode, the internal polarization is generated. Thus, when the internal polarization is generated, voltage applied to the liquid crystal layer is adversely affected. In other words, the internal polarization superposes a DC offset voltage on an AC voltage that is applied between the picture element electrode and the opposite electrode in order to change the orientation of liquid crystal molecules, thereby putting the AC voltage in the non-equilibrium state. In such a non-equilibrium state, flickering occurs generated on the display picture plane, which causes deterioration in contrast. Moreover, the variation in the superposed DC component causes a contrast ununiformity, which remarkably lowers the display quality. Intensity of such an internal polarization, when the insulating protective film is held between the polyimide resin orientation film and the picture element electrode, is affected by the thickness of the insulating protective film, and it increases with an increase in the thickness of the insulating protective film. In the case where a polyimide resin is used as the insulating protective film in the same manner as that of the liquid crystal display apparatus disclosed in the aforesaid Publication, a film thickness of 2 .mu.m or more is required, whereby, even when the insulating protective film is laminated only on part of the picture element electrode, there is a deterioration of display quality, such as contrast unevenness, caused by the generation of the internal polarization.
When TFT is used as the function element connected to each picture element electrode, a gate voltage that is generated by capacitance formed between a gate electrode and a drain electrode couples with a drain voltage to thereby superpose the DC component on the drain voltage. The DC component is compensated to a certain extent by applying the DC component to the opposite electrode. However, the DC component superposed on the drain electrode largely changes due to the source voltage, so that it cannot be completely compensated. Such an incompletely compensated DC component is applied to the orientation film of a polyimide resin, so that the aforesaid internal polarization further increases. Thus, it takes a long time for the large internal polarization generated by a source voltage applied as the image signal to disappear, and accordingly the internal polarization is stored for a predetermined time. As a result, the orientation of liquid crystal molecules cannot follow a change in the source voltage (i.e., image signal), which results in residual images on the display picture plane. In the active matrix type liquid crystal apparatus using TFT as the function element, the internal polarization is one factor that causes the residual images.
As mentioned above, in the case where the orientation film of a polyimide resin is laminated on the insulating protective film disposed on the picture element electrode the generation of internal polarization creates various problems, but when the orientation film formed of an inorganic material, such as SiO.sub.2, or of a silane coupling agent is used, the aforesaid problem is scarcely created.
When the polyimide resin is used as the orientation film, the aforesaid problem is created by the insulating protective film laminated on the picture element electrode. To solve this problem, it was considered to form no insulating protective film. However, when the insulating protective film is not formed, the picture element electrode substrate is apt to suffer mechanical and chemical damage when manufactured. For example, when the orientation film of a polyimide resin is subjected to a rubbing process, the film of the picture element electrode or the like is easy to peel off. Also a leakage current between the respective picture element electrodes increases. For example, a TFT off-current increases to cause a display unevenness, thereby deteriorating the display quality and also the reliability of the display apparatus.